Reducing Rpm Through Gear Splitter?

droptail · August 19, 2009

Since GM (or anybody else) isn't giving LD configurations (RC/SB) any new increased mileage technologies (LD diesel, cylinder deactivation, hybrid drivetrain, 6-speed trans, etc.) I guess I have to pursue it myself.

1) Any mpg data on lowering a cruising rpm from about 2100 to 1650?

2) Any experience using a splitter, particularly in a LD application?

Thanks

2COR517 · August 19, 2009

When you say LD, you mean light duty?

What speed do you have that RPM at cruise? Are you talking about the truck in your sig? And have you recalibrated your speedo for your gears and tires? For what a splitter/OD would cost you, you can buy ALOT of fuel.

WaterGuy · August 20, 2009

Lowering the RPM is not always the answer.

Engines work at various "sweet spots".

It could be that the RPM is too low and the engine is working too hard thus not getting good MPG.

Gotta find someone smarter than me to get to the next level of play.

droptail · August 20, 2009

When you say LD, you mean light duty?
What speed do you have that RPM at cruise? Are you talking about the truck in your sig? And have you recalibrated your speedo for your gears and tires? For what a splitter/OD would cost you, you can buy ALOT of fuel.

Yes, light duty truck in my sig.

70mph cruise (want to go 75ish though).

Stock gears and tire diameter.

I would benefit from less shifting from the gear skipping, which I do now but isn't so smooth. I've been shifting this bitch for 20 years and as much as I like a 5-speed, it gets old when there's stop & go traffic (and the tranny/shifter is shitty).

I plan to buy it used at a substantial savings.

Bottom line, it has to give me at least 3 mpg improvement or its not worth it. If I can eek out 20 mpg, I will be happy.

Waterguy, I know that, thats why I'm asking for anyones' experience with this configuration. Maybe I can't get an improvement, only someone who has tried it or has tall gears to get above rpm, will know for sure.

I do want to go back to my Holley TBI after I mod it, and perhaps roller rockers with increased ratio, and run that if mileage doesn't drop.

2COR517 · August 20, 2009

I don't think you're going to have any problem getting 20mpg from a few year old RC/SB. Start modding it, and all bets are off. Custom tune is the way to go, better performance and usually better economy. Cheap too.